Your Sky Help: Sky Map Control Panel

This document contains all the controls in Your Sky's Sky Map
control panel, with an explanation of the function of each. The
controls in this document are “live”—you're free to experiment with
various settings, pressing the “Update” button at the top or bottom of
the page to display a star map with the settings you've chosen.

Three options are available for specifying the date and time.
Choose an option by checking the box to the left and
(for options other than “Now”) enter the date in the
corresponding format in the text box to the right.

This option permits you to enter any Universal (also known as Greenwich Mean)
date and time in the format
year/month/dayhour/minute/second.
You can omit the minutes and seconds of the time, or omit the
time entirely if you wish to specify 00:00 UTC.

You can enter dates as far back as Julian day 0, January 1, −4712
and as far into the future as you wish. Note that astronomers and
historians use different conventions for years before A.D. 1. In
history books, the year that preceded A.D. 1 is called 1 B.C., zero
not having come into use in European culture at the time.
Astronomers denote the year before A.D. 1 as “year 0”. Thus when
an astronomer talks about an eclipse having occurred in the year
−412, that's the year historians refer to as “413 B.C.”. In
converting historical dates to Julian days, Your Sky assumes the
canonical date for the adoption of the Gregorian calendar, Friday,
October 15th, 1582. Many countries shifted to the Gregorian
calendar much later; in Great Britain, not until 1752. When
investigating events in history, make sure you express all dates
after October 15th, 1582 in the Gregorian calendar.

Astronomers often have to do arithmetic with dates and times. The
Gregorian calendar is sufficiently eccentric that answering a
question like “What is the date and time 295.03589 days (10 lunar
months) from now?” is a nontrivial exercise. To facilitate
computation, astronomers employ the Julian day calendar.
The Julian day number for a given moment in time is simply the
number of days, whole and fractional, elapsed since noon (12:00)
Universal time on 1st January −4712; time is expressed as a
fraction of a day. This system allows assigning a positive number
to the date and time of any observation in recorded history and
arbitrarily far into the future, and permits ordinary arithmetic
with dates and times without having to worry about B.C. and A.D.,
Julian and Gregorian calendars, leap years, and all that. Of
course, you need to be able to convert back and forth between
Julian days and the civil calendar, but that's what computers are
for.

To show a star map for a given Julian day, simply check the box
and enter the Julian day number in the box to the right, including
a fraction where appropriate. (Be sure to remember that Julian
days begin at noon—a Julian date representing midnight will end
in “.5”.) Click “Update” to show the map for the designated
observing site at that day.

Obviously, what you see in the sky depends on where you're
standing on the Earth. Your Sky, therefore, needs to know the
latitude and longitude of your observing site (or other
location for which you wish a star map) in order to
correctly display the view from there. Enter
the latitude and longitude in the boxes below (the
values in the boxes are those for Boston, on the East
coast of North America). For each value, enter the
degrees and, optionally, minutes and seconds, and
be sure to check the correct North/South and
East/West box for each setting. You can enter values
using either the ISO degree sign, “░”, (which may not be on
your keyboard), or with a “d” denoting degrees; Boston's
latitude could have been entered as 42d21m24s.

If you don't know the latitude and longitude of your observing
site, or you wish to display the sky as seen from various
locations on Earth, the link above will take you to a list
of various places on Earth, mostly cities, which, when clicked,
cause Your Sky to show a star map above that location at the
current date and time. If you wish to display the map for
a different time, enter it in the Universal time box in the
control panel below the map and re-submit the request with
the Universal time box checked.

This box controls whether the celestial coordinate system appears
in the map. When checked, the pole is marked with a small light
blue cross with arms pointing toward the 0, 6, 12, and 18 hour
marks on the equator, the celestial equator is drawn in light
blue, labeled at each hour of right ascension, and the ecliptic is
drawn in red with labels every 15░ of ecliptical longitude.

The celestial poles are the points in the sky at the zenith
above the Earth's poles; the celestial equator is the projection
of the Earth's equator onto the sky, and the ecliptic is the
plane in which the Earth orbits the Sun. The obliquity of
the ecliptic changes slowly through time as the Earth's
inclination varies, and the points at which the ecliptic crosses the equator
(the equinoxes) also shift over time due to
precession.

If this box is checked, the Moon and planets will be included
in the map at their correct positions for the given time and date.
The Moon icon will show the Moon with the correct phase, with
the lunar north pole at the top of the icon. A table of
planet names and icons is
given in an accompanying document.

The technique used to calculate the positions of the planets
from Mercury through Neptune is
valid only for dates less than A.D. 8000; if you specify a date further into
the future, the Moon and planets are not plotted on the map and no
ephemeris is shown. Pluto has been
observed over an insufficient arc of its orbit to allow
accurate computation of its position outside the range of
years from 1885 through 2099; Pluto is not plotted for dates
outside this interval.

Checking this box causes deep sky objects (galaxies, star clusters,
gaseous nebulŠ, etc.) brighter (in integrated visual intensity) than
the given magnitude to be plotted in the star map, using
icons to distinguish the
different types of objects.

Here we've made a finder chart to locate M31, the great spiral
galaxy in Andromeda, labeling brighter stars with their Bayer
and Flamsteed designations.

The human visual system, inherited from hundreds of millions of
years of mammalian evolution, excels in finding patterns. Spotting
the fearful symmetry of a tiger lurking in the bush quickly enough
to run away means you're likely to have more children than
folks who lack that talent, so we've all inherited the genes of those
with the high-end pattern matching meatware. As a result,
we see patterns everywhere, even where no pattern
is really there. What child, or adult on a lazy day, hasn't
gazed at the clouds boiling up on a hot summer afternoon
and seen, in the sky, the fantasy images which inhabit our
dreams?

We see patterns in the night sky as well. Every civilisation
has grouped the stars into constellations representing
key aspects of their culture. The constellations in the
northern hemisphere sky recognised by Western cultures
are largely inherited from classical Greece—they are the
goddesses and gods of Olympus and the heroes of Greek
mythology.

Constellations in the southern hemisphere were named innumerable
times by the multitude of cultures who observed them since
antiquity. European culture was, however, unaware of most
of the southern sky prior to the Enlightenment and the
ensuing age of exploration, so many of them were named for
contemporary wonders such as the microscope (Microscopium),
air pump (Antila), and clock (Horologium).

Perhaps if our cultural baggage had been lost at the
generation-port, we'd look at the stars and see in
them the Five Original Marx Brothers, Lucy in the Sky,
and Bart's Skateboard. Your Sky serves up the traditional
constellations, offering the following options.

If checked, each constellation's name is shown
in yellow, centred on the midpoint of the constellation.

aligned with horizon?

By default, constellation names are shown left to right. If this
box is checked, names are aligned with the horizon as usually
done in printed star maps. Aligning names minimises truncation of
names of constellations near the horizon, but may be more difficult
to read on computer monitors.

If checked, the boundaries between adjacent constellations are drawn
in green.

The constellation boundaries we use today were adopted in 1930 by
the International Astronomical Union,
based on a partial set of
boundaries compiled in 1877 by B. A. Gould. Gould's original boundaries
star maps but, for simplicity, ran purely east-west and
north-south in the celestial sphere. But as the Earth's axis
precesses
with regard to the distant stars, completing a full
circle every 25,800 years, equatorial coordinates change with
regard to the fixed stars, so the original boundaries, defined
in 1877, no longer run parallel to lines of
longitude and latitude today. (Precession may seem like
a minor effect, significant only in the very long term, but it can
creep up on you. In the century and a quarter since 1877,
precession has moved Polaris three quarters of a degree closer to
the north celestial pole—that's one and a half
diameters of the full Moon!)

Consequently, it's necessary to adjust the constellation
boundaries to account for precession. The boundaries used by
Your Sky have been precessed in a simple fashion to the
J2000.0 epoch. Approximating the precise 1875 boundaries based on
the best available published data would require plotting more than
13,000 vectors and didn't seem worth it, especially since you
could hardly see the difference on an all-sky map.

The following controls permit choosing how many stars
will appear in the map and select the annotation which
accompanies them. Each includes a limiting magnitude
field. The brightness of a star is given by its
magnitude, with a larger numerical magnitude indicating
a dimmer star. The brightest star (excluding the Sun) is
Sirius, with a magnitude of −1.46. There are about 20 stars
brighter (having a magnitude less than) 1.5, and about 100
stars brighter than magnitude 2.6. The number of stars increases
rapidly with the magnitude—there are about a thousand stars
brighter than magnitude 4.5, and if you use magnitude 5.5 as
the limit for naked eye visibility, you include about 3000
stars. Your Sky's Sky Map uses the Yale Bright Star
Catalogue, Fifth Edition, which includes more than 9000
stars brighter than magnitude 6.5.

Only stars brighter than specified by this box will be
included in the map. Stars are shown as icons; the larger
the icon, the brighter the star; due to resolution limits
of computer monitors (and convention in many printed star
atlases), stars are “binned” into units of one full magnitude.

The following controls select the annotation which
accompanies plotted star icons.
Note that the annotation settings in the items which follow apply
only to stars which are plotted; regardless of the limiting
magnitude for an annotation, it will never appear unless
the star it applies to is brighter than the cutoff for
the map given above.

Many bright stars are named, for example “Polaris”, “Altair”, and
“Zubenelgenubi”, and principal stars of constellations are
designated by Greek letters often called “Bayer letters” after
Johann Bayer who first identified stars this way in his
Uranometria of 1603. Multiple stars may bear the same
letter and be distinguished by a numeric subscript. Stars are
also identified by “Flamsteed numbers”, which simply number the
stars within a constellation in order of right ascension.
Stars in southern constellations are frequently identified by
Roman letters.

If this box is checked, named stars brighter than the given
magnitude will show the name to the right of the star icon in
the map. If you set the magnitude too high, the map may become
cluttered with names and difficult to read.

Checking this box causes the Bayer, Flamsteed, or letter
designation to be shown for stars brighter than the given
magnitude, plotted to the right of the star unless its name
is also shown, in which case the code is shown to the left.

Your Sky draws sky maps following the usual convention that
maps for observing sites in the northern hemisphere are plotted
with North up, while maps for observers in the southern
hemisphere have South up. Checking the above box inverts the
map from this default orientation.

This field specifies the size, in pixels, of the sky map.
Map size is restricted to the range of 100 to 1024
pixels; smaller maps would be useless, and larger maps take
too long to generate and download.

The text used to label the chart will be scaled by the specified
factor, with 1.0 the default. Scale factors less than one reduce the size
of the text while those greater than one enlarge it compared to the
default size.

You can select one the following colour schemes for the sky map.
Samples of each colour scheme are given below, along with a discussion
of the applications to which it is suited.

Colour

The default full colour scheme is easiest to view on computer
monitors which support 256 (or more) colours. Colour coding
coordinates, constellation boundaries and outlines, annotation,
and planet icons makes it easier to distinguish the various
objects, especially when the map is crowded with many different
items.

Black on white background

This is the traditional choice for printed star atlases.
If you're planning to print the chart on a black and white
printer, this is usually the best choice. The white background
allows you to write on the map, and keeps the background from
using up lots of ink or toner in your printer.

White on black background

Many astronomers find charts with white stars and text on a
black background easier to read with the dim red flashlights
they use at the telescope to preserve night vision. Your Sky
will produce sky maps in this form, but if you're planning
to print the resulting charts, ponder the
consequences for your printer. All of that black has to come
from somewhere, and that somewhere is generally your printer's
ink or toner cartridge. Making lots of white on black maps
can use up cartridges at a prodigious rate compared to
printing normal text. Also, mostly black documents
cause problems with some printers; laser printers may
print a “shadow” on subsequent pages due to excess toner
adhering to the imaging surface, and inkjet printers sometimes
splatter ink around when feeding the large amount needed for
extended regions of black. Your printer may be immune from
these foibles, but if it isn't, don't say I didn't warn you.

Night vision (red)

If you're taking your laptop computer into the field to use
at the telescope, this option may be just the ticket. All
items on the map are displayed in red, which doesn't tend
to degrade night vision. When using this option, use the
display brightness control to reduce the intensity to the
minimum level at which you can still easily read the map.
Where possible, other material in the map document is
displayed red on black, like this paragraph.

Given the orbital elements for an asteroid (minor planet)
or comet, Your Sky can calculate the position of that object
in the sky for any time within the epoch of validity
of the elements and display it in the correct location on
the map. Specifying orbital elements is an advanced topic
which is discussed in detail in a separate
Asteroid and Comet Tracking
document.